STUDIES IN THE EMBRYOLOGY OF ECHINODERMS. 421 
may be so, but the evidence seems to me to point rather to 
the water-pore having existed in a very early stage in the 
history of Echinoderms, probably before the hydrocoel had 
arisen. 
A word must be said here as to the probable function of the 
water-pore. Bearing in mind Hartog’s experiments and 
remarks (12) upon the madreporite of the adult, I carefully 
observed not only the apparent motion of the cilia but also the 
action of the currents produced by them, as indicated by the 
motion of particles suspended in the water. The apparent 
motion of the cilia was inwards, which, as we know, indicates 
that the real current produced is exhalent. If we need an 
illustration of this we have only to turn from the water-pore to 
the oesophagus, and observe the motion there ; for in this case, 
while the apparent motion is outwards, particles suspended in 
the water show clearly that the current passes inwards. With 
regard to the water-pore also particles suspended in the water 
will guide us as to the direction of the current, though not 
so readily as in the case of the oesophagus. No particles were 
ever observed to pass in through the pore, though there was 
nothing in their size to prevent them ; on the other hand, it 
was difficult to observe an exhalent current owing to the rapid 
motion imparted to the particles by the external cilia of the 
larvae; nevertheless, in a few cases in Echinid Piute i, in 
Bipinnaria, in Auricularia, and in Tornaria, a definite repulsion 
of particles from the pore was noticed. Taking this in connec- 
tion with the apparent motion of the cilia (also observed in 
Ophiurid Plutei) it seems safe to assert that the current 
passing through the water-pore is an exhalent one, though, 
from the very slight disturbances produced by it in the sur- 
rounding water, I conclude that it is not usually very strong. 
It is, of course, not proved that the current is never inhalent; 
but until such a reversal of its direction has been definitely 
observed we have no particular reason for supposing that it 
ever occurs. 
As many of the larvae observed by me had as yet no hydrocoel, 
we arrive at the conclusion that in Echinoderms as well as in 
